MOLECULAR-DYNAMICS STUDY OF SCANNING FORCE MICROSCOPY ON SELF-ASSEMBLED MONOLAYERS

To understand scanning force microscope (SFM) images with molecular re solution on organic carpet-like films, we have performed isothermal mo lecular dynamics simulations of CH3(CH2)(10)S self-assembled monolayer s (SAMs) on Au(111) within the ''united-atoms'' model. The SFM is repr esented by a deformable pyramidal cluster connected by orthogonal spri ngs to a rigid support. Most of the relevant time scales (scanning, in strument response, SAM evolution) have been separated. With increasing penetration, continuous sliding is followed by stick-slip, leading to net friction, either extrinsic, i.e. governed by the soft spring(s) o f the instrument. or intrinsic. Molecules close to the tip are strongl y deformed, and the simulated SAM domain can be collectively tilted to wards the scan direction. Defects in the chemisorbed S-layer are forme d and are dragged beyond a critical load. Our results provide insight into the molecular origin of friction and viscoplastic response of mod el lubricant films actively investigated from both fundamental and app lied viewpoints.